37 CFR § 42.406 - Service of petition.
(a) The petition and supporting evidence must be served on the respondent at the correspondence address of record for the earlier application or subject patent. The petitioner may additionally serve the petition and supporting evidence on the respondent at any other address known to the petitioner as likely to effect service.
(b) Upon agreement of the parties, service may be made electronically. Service may be by Priority Mail Express® or by means at least as fast and reliable as Priority Mail Express®. Personal service is not required.